Does the solar term mean one day or fifteen days later?

In calendar science, solar terms are actually a general term for festival and harmony. The number of days of the festival is 15 days. It is fifteen days from the time of the festival to the day you step into the air, or fifteen days from the day you step into the festival. For example, it takes fifteen days from the day of beginning of spring (solar term) to the day of rain (standing gas), and it takes fifteen days from the day of rain (standing gas) to the next solar term (fright).